The field relates to two-stroke cycle, opposed-piston engines. Particularly, the field concerns a compact engine structure for an opposed-piston engine with a split cylinder block. The term “engine structure” is taken to mean an assembly including a cylinder block and associated crankcases. Further, a “crankcase” is a housing with a crankshaft and its associated main bearings.
A two-stroke cycle engine is an internal combustion engine that completes a cycle of operation with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. The strokes are typically denoted as compression and power strokes. One example of a two-stroke cycle engine is an opposed-piston engine in which two pistons are disposed in the bore of a cylinder for reciprocating movement in opposing directions along the central axis of the cylinder. Each piston moves between a bottom center (BC) location where it is nearest one end of the cylinder and a top center (TC) location where it is furthest from the one end. The cylinder has ports formed in the cylinder sidewall near respective BC piston locations. Each of the opposed pistons controls one of the ports, opening the port as it moves to its BC location, and closing the port as it moves from BC toward its TC location. One of the ports serves to admit charge air into the bore, the other provides passage for the products of combustion out of the bore; these are respectively termed “intake” and “exhaust” ports (in some descriptions, intake ports are referred to as “air” ports or “scavenge” ports).
FIG. 1 illustrates a two-stroke cycle, opposed-piston engine 10. The engine 10 has a plurality of ported cylinders, one of which is indicated by reference numeral 50. For example, the engine may have two ported cylinders, or three or more ported cylinders. Each ported cylinder 50 has a bore 52 and longitudinally-spaced intake and exhaust ports 54 and 56 formed or machined near respective ends of a cylinder wall. Each of the intake and exhaust ports includes one or more circumferential arrays of openings or perforations. In some descriptions, each opening is referred to as a “port”; however, the construction of one or more circumferential arrays of such “ports” is no different than the port constructions shown in FIG. 1. Pistons 60 and 62 are slidably disposed in the bore 52 with their end surfaces 61 and 63 in opposition. The piston 60 controls the intake port 54, and the piston 62 controls the exhaust port 56. In the example shown, the engine 10 further includes two crankshafts 71 and 72. The intake pistons 60 of the engine are coupled to the crankshaft 71, and the exhaust pistons 62 to the crankshaft 72.
As the pistons 60 and 62 near their TC locations in the cylinder 50, a combustion chamber is defined in the bore 52 between the end surfaces 61 and 63 of the pistons. Fuel is injected directly into the combustion chamber. In some instances injection occurs at or near minimum volume (the point in the compression cycle where minimum combustion chamber volume occurs because the pistons end surfaces are nearest each other); in other instances, injection may occur before minimum volume. Fuel is injected through one or more fuel injector nozzles positioned in respective openings through the sidewall of the cylinder 50. Two such nozzles 70 are shown. The fuel mixes with charge air admitted into the bore 52 through the intake port 54. As the air-fuel mixture is compressed between the end surfaces 61 and 63, the compressed air reaches a temperature and a pressure that cause the fuel to ignite. Combustion follows.
With further reference to FIG. 1, the engine 10 includes an air handling system 80 that manages the transport of charge air to, and exhaust gas from, the engine 10. A representative air handling system construction includes a charge air subsystem 81 and an exhaust subsystem 82. In the air handling system 80, a charge air source receives intake air and processes it into pressurized air (hereinafter “charge air”). The charge air subsystem 81 transports the charge air to the intake ports of the engine. The exhaust subsystem 82 transports exhaust products from exhaust ports of the engine for delivery to other exhaust components. In some aspects, the air handling system 80 may be constructed to reduce undesirable emissions produced by combustion by recirculating a portion of the exhaust gas produced by combustion through an exhaust gas recirculation (“EGR”) system 83. The recirculated exhaust gas is mixed with charge air to lower peak combustion temperatures, which reduces production of the undesirable emissions.
With reference to FIG. 2, an engine structure for a two-stroke cycle, dual-crankshaft, opposed-piston engine 90 includes a cylinder block 100, a crankcase assembly 102, and a crankcase assembly 104. The cylinder block 100 includes a plurality of cylinders 106 aligned in a row such that a single plane bisects, and contains the longitudinal axes of, all of the cylinders. The row-wise alignment of the cylinders 106 is referred to as an “inline” configuration in keeping with standard nomenclature of the engine arts. Furthermore, the inline arrangement can be “straight”, wherein the plane containing the longitudinal axes is essentially vertical, or “slant”, wherein the plane containing the longitudinal axes is slanted. It is also possible to position the engine in such a manner as to dispose the plane containing the longitudinal axes essentially horizontally, in which case the inline arrangement would be “horizontal”. Thus, while the following description is limited to an inline configuration, it is applicable to straight, slant, and horizontal variations.
In this specification, a “cylinder” is taken to be constituted of a liner (sometimes called a “sleeve”) retained in a cylinder tunnel formed in the cylinder block 100. The inline array of cylinders 106 is aligned with an elongate dimension L of the cylinder block 100. Taking the left-most cylinder 106 to be representative of all of the cylinders 106, each cylinder has a bore 152 and an annular intake portion including an intake port 154 separated along the longitudinal axis of the cylinder from an annular exhaust portion including an exhaust port 156. The end of the cylinder nearest the intake port 154 is referred to as the “intake end” of the cylinder, and the end nearest the exhaust port 156 is referred to as the “exhaust end”. The cylinders 106 are arranged such that their intake and exhaust ends are aligned in respective sides of the inline array. Two counter-moving pistons 160 and 162 are disposed in the liner bore of each cylinder. The pistons 160 control the intake ports of the engine; the pistons 162 control the exhaust ports. A crankshaft 171 is rotatably supported by main bearings B1 along the intake end of the cylinders 106, in parallel alignment with the elongate dimension L. All of the pistons 160 are coupled to the crankshaft 171. A crankshaft 172 is rotatably supported by main bearings B2 along the intake end of the cylinders 106, in parallel alignment with the elongate dimension L. All of the pistons 162 are coupled to the crankshaft 172. The crankshafts 171 and 172 are coupled by a gear train 175, or by other equivalent means including one or more of a beveled gear drive, a belt, and a chain.
The crankcase assembly 102 includes the crankshaft 171 and the main bearings B1. The crankcase assembly 104 includes the crankshaft 172 and the main bearings B2. The engine structure may also include a gear box 105 housing the gear train 175. In such a case, the gear box 105 may extend over a face of the cylinder block 100, between the crankcase assemblies 102 and 104.
The inline, dual-crankshaft engine structure shown in FIG. 2 differs substantially from the standard inline and V structures of two- and four-stroke engines in which each cylinder contains only a single piston and all pistons are connected to a single crankshaft. The structural differences are especially in evidence when considering the difficulty of fitting the two-stroke cycle opposed-piston engine structure of FIG. 2 to vehicle engine compartment space configured for standard inline and V engine structures. In this regard, see related application U.S. application Ser. No. 14/028,423. Further, even when not constrained by predetermined engine compartment configurations, the opposed-piston engine structure of FIG. 2 can be difficult to fit to a vehicle. Therefore, it is important to make the opposed-piston engine structure as compact as possible so as to occupy minimal space in applications such as vehicles, locomotives, maritime vessels, stationary power sources, and so on.
As per FIG. 2, one step in achieving a compact engine structure for the illustrated engine is to minimize the center-to-center spacing between the cylinders 106 so as to reduce the elongate dimension L of the cylinder block 100. There are, however, at least two impediments to this solution. First, the high pressures produced during combustion may lead to constructions that strengthen the cylinders, especially around the cylinder zones where the pistons are at or near TC. As seen in FIG. 3, this can lead to a cylinder structure that includes a liner 200 equipped with a compression sleeve 202 configured with intake and exhaust ports 203 and 205, respectively, girding an intermediate liner portion between the cylinder's intake and exhaust ends 204 and 206. These parts share a common longitudinal axis 207. The compression sleeve 202 results in an outer diameter DM in the intermediate portion of the liner that is larger than the outer diameter DE of the two ends 204 and 206. The second impediment is raised by provision of a bearing web structure capable of withstanding the forces applied to the cylinder block by the main bearings. In the bearing web structure of FIG. 2 the web elements 180 (sometimes called “bearing partitions”) extend from main bearings B1 to main bearings B2, passing between the cylinders 106. In view of these elements, the minimum center-to-center cylinder bore spacing is greater than the sum of the diameter DM of a compression sleeve 202 (FIG. 3) and the thickness of a bearing web member 180 (FIG. 2).